Can a hidden Zhonya be revealed as a reaction to save a unit that is being moved or discarded from a battlefield?

Ruling: Yes, hidden Zhonya can be revealed as a reaction to save units being moved or discarded. Hidden cards have Reaction speed, allowing them to react to any action, and when a gear at a battlefield is revealed it returns to base.

Sequence:

• Opponent takes an action that would move or discard your unit (e.g., Charm, Flight or Fight)
• You can react by revealing the hidden Zhonya
• Zhonya returns to your base
• The unit is saved from being moved/discarded

Nuances:

• This works even if the unit has already been moved to a different battlefield before you react
• A hidden Zhonya at one battlefield (BF1) can be revealed during a showdown at a different battlefield (BF2) to save a unit at BF2